What is the local mean age of air, đĚ p?
The nominal time constant đĚ đ is defined as:
The nominal time constant đĚ đ for a room with piston flow:
air change efficiency Îľa
contaminant removal effectiveness Îľc
displacement ventilation
introduced through floor terminals or other devices located near the floor
The supply air is colder than the room air, the supply air falls down to the floor level and distributes evenly.
The room contains a heat source (people, equipment, heaters, etc.), and the air temperature rises near the heat source and transports contaminations to a higher horizontal layer in the room.
The air is then exhausted from the room near the ceiling
If the supply air temp. is not lower than the room temp., short circuiting can occur.
The main advantage of displacement ventilation is that the concentration of pollutions will be lower in the occupied zone than in the exhaust
airflow rate can be reduced compared to mixing ventilation and still maintain good air quality.
Disadvantages is that the temperature will be lower close to the inlet.
Mixing ventilation
the air is supplied to the room by jets.
Air temperature and concentration of contaminants are more or less uniformly distributed (diluted) in the room and nearly the same as in the exhaust air.
the supply air devices are typically placed in the ceiling, outside the zone of occupancy.
The advantage of mixing ventilation is that cold air can be supplied without any risk for draft.
Mixing ventilation will give an air change efficiency of 50 %.
Last changed2 years ago
